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Researchers have discovered the first cases of whooping cough in the U.S. caused by a germ that may be resistant to the vaccine, according to a recent report in the New England Journal of Medicine.

Scientists from the U.S. Centers for Disease Control and Prevention and other researchers looked into a dozen vaccine-resistant cases of whooping cough, also known as pertussis, in Philadelphia. The new resistant strain was previously reported in Finland, Japan and France, Associated Press reports.

The new strain could be one factor contributing to the worst year in the U.S. for whooping cough in six decades.

"It's quite intriguing," Tom Clark, a pediatrician with the CDC, said, according to Associated Press. "It's the first time we've seen this here. We definitely need some more information about this before we can draw any conclusions."

The study suggests that the new strain is not more deadly, but that shots may not work as effectively against it.

Pertussis is a highly contagious disease that can infect people of all ages, though the disease is more dangerous for children. Pertussis used to be common, but case numbers dropped after a vaccine was introduced in the 1940s.

An increase in the number of cases in recent years was partially blamed on a version of the vaccine used since the 1990s that does not last as long.